Extended statistics on expressions

Allow defining extended statistics on expressions, not just just on
simple column references.  With this commit, expressions are supported
by all existing extended statistics kinds, improving the same types of
estimates. A simple example may look like this:

  CREATE TABLE t (a int);
  CREATE STATISTICS s ON mod(a,10), mod(a,20) FROM t;
  ANALYZE t;

The collected statistics are useful e.g. to estimate queries with those
expressions in WHERE or GROUP BY clauses:

  SELECT * FROM t WHERE mod(a,10) = 0 AND mod(a,20) = 0;

  SELECT 1 FROM t GROUP BY mod(a,10), mod(a,20);

This introduces new internal statistics kind 'e' (expressions) which is
built automatically when the statistics object definition includes any
expressions. This represents single-expression statistics, as if there
was an expression index (but without the index maintenance overhead).
The statistics is stored in pg_statistics_ext_data as an array of
composite types, which is possible thanks to 79f6a942bd.

CREATE STATISTICS allows building statistics on a single expression, in
which case in which case it's not possible to specify statistics kinds.

A new system view pg_stats_ext_exprs can be used to display expression
statistics, similarly to pg_stats and pg_stats_ext views.

ALTER TABLE ... ALTER COLUMN ... TYPE now treats indexes the same way it
treats indexes, i.e. it drops and recreates the statistics. This means
all statistics are reset, and we no longer try to preserve at least the
functional dependencies. This should not be a major issue in practice,
as the functional dependencies actually rely on per-column statistics,
which were always reset anyway.

Author: Tomas Vondra
Reviewed-by: Justin Pryzby, Dean Rasheed, Zhihong Yu
Discussion: https://postgr.es/m/ad7891d2-e90c-b446-9fe2-7419143847d7%40enterprisedb.com

1 files changed, 273 insertions, 164 deletions

diff --git a/src/backend/commands/statscmds.c b/src/backend/commands/statscmds.c
index 2bae2058459..df4768952d5 100644
--- a/src/backend/commands/statscmds.c
+++ b/src/backend/commands/statscmds.c
@@ -29,6 +29,8 @@
 #include "commands/comment.h"
 #include "commands/defrem.h"
 #include "miscadmin.h"
+#include "nodes/nodeFuncs.h"
+#include "optimizer/optimizer.h"
 #include "statistics/statistics.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
@@ -62,7 +64,8 @@ ObjectAddress
 CreateStatistics(CreateStatsStmt *stmt)
 {
 	int16		attnums[STATS_MAX_DIMENSIONS];
-	int			numcols = 0;
+	int			nattnums = 0;
+	int			numcols;
 	char	   *namestr;
 	NameData	stxname;
 	Oid			statoid;
@@ -74,21 +77,25 @@ CreateStatistics(CreateStatsStmt *stmt)
 	Datum		datavalues[Natts_pg_statistic_ext_data];
 	bool		datanulls[Natts_pg_statistic_ext_data];
 	int2vector *stxkeys;
+	List	   *stxexprs = NIL;
+	Datum		exprsDatum;
 	Relation	statrel;
 	Relation	datarel;
 	Relation	rel = NULL;
 	Oid			relid;
 	ObjectAddress parentobject,
 				myself;
-	Datum		types[3];		/* one for each possible type of statistic */
+	Datum		types[4];		/* one for each possible type of statistic */
 	int			ntypes;
 	ArrayType  *stxkind;
 	bool		build_ndistinct;
 	bool		build_dependencies;
 	bool		build_mcv;
+	bool		build_expressions;
 	bool		requested_type = false;
 	int			i;
 	ListCell   *cell;
+	ListCell   *cell2;
 
 	Assert(IsA(stmt, CreateStatsStmt));
 
@@ -190,101 +197,124 @@ CreateStatistics(CreateStatsStmt *stmt)
 	}
 
 	/*
-	 * Currently, we only allow simple column references in the expression
-	 * list.  That will change someday, and again the grammar already supports
-	 * it so we have to enforce restrictions here.  For now, we can convert
-	 * the expression list to a simple array of attnums.  While at it, enforce
-	 * some constraints.
+	 * Make sure no more than STATS_MAX_DIMENSIONS columns are used. There
+	 * might be duplicates and so on, but we'll deal with those later.
+	 */
+	numcols = list_length(stmt->exprs);
+	if (numcols > STATS_MAX_DIMENSIONS)
+		ereport(ERROR,
+				(errcode(ERRCODE_TOO_MANY_COLUMNS),
+				 errmsg("cannot have more than %d columns in statistics",
+						STATS_MAX_DIMENSIONS)));
+
+	/*
+	 * Convert the expression list to a simple array of attnums, but also keep
+	 * a list of more complex expressions.  While at it, enforce some
+	 * constraints.
+	 *
+	 * XXX We do only the bare minimum to separate simple attribute and
+	 * complex expressions - for example "(a)" will be treated as a complex
+	 * expression. No matter how elaborate the check is, there'll always be a
+	 * way around it, if the user is determined (consider e.g. "(a+0)"), so
+	 * it's not worth protecting against it.
 	 */
 	foreach(cell, stmt->exprs)
 	{
 		Node	   *expr = (Node *) lfirst(cell);
-		ColumnRef  *cref;
-		char	   *attname;
+		StatsElem  *selem;
 		HeapTuple	atttuple;
 		Form_pg_attribute attForm;
 		TypeCacheEntry *type;
 
-		if (!IsA(expr, ColumnRef))
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("only simple column references are allowed in CREATE STATISTICS")));
-		cref = (ColumnRef *) expr;
-
-		if (list_length(cref->fields) != 1)
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("only simple column references are allowed in CREATE STATISTICS")));
-		attname = strVal((Value *) linitial(cref->fields));
-
-		atttuple = SearchSysCacheAttName(relid, attname);
-		if (!HeapTupleIsValid(atttuple))
-			ereport(ERROR,
-					(errcode(ERRCODE_UNDEFINED_COLUMN),
-					 errmsg("column \"%s\" does not exist",
-							attname)));
-		attForm = (Form_pg_attribute) GETSTRUCT(atttuple);
-
-		/* Disallow use of system attributes in extended stats */
-		if (attForm->attnum <= 0)
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("statistics creation on system columns is not supported")));
-
-		/* Disallow data types without a less-than operator */
-		type = lookup_type_cache(attForm->atttypid, TYPECACHE_LT_OPR);
-		if (type->lt_opr == InvalidOid)
+		/*
+		 * We should not get anything else than StatsElem, given the grammar.
+		 * But let's keep it as a safety.
+		 */
+		if (!IsA(expr, StatsElem))
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("column \"%s\" cannot be used in statistics because its type %s has no default btree operator class",
-							attname, format_type_be(attForm->atttypid))));
+					 errmsg("only simple column references and expressions are allowed in CREATE STATISTICS")));
 
-		/* Make sure no more than STATS_MAX_DIMENSIONS columns are used */
-		if (numcols >= STATS_MAX_DIMENSIONS)
-			ereport(ERROR,
-					(errcode(ERRCODE_TOO_MANY_COLUMNS),
-					 errmsg("cannot have more than %d columns in statistics",
-							STATS_MAX_DIMENSIONS)));
+		selem = (StatsElem *) expr;
 
-		attnums[numcols] = attForm->attnum;
-		numcols++;
-		ReleaseSysCache(atttuple);
+		if (selem->name)		/* column reference */
+		{
+			char	   *attname;
+
+			attname = selem->name;
+
+			atttuple = SearchSysCacheAttName(relid, attname);
+			if (!HeapTupleIsValid(atttuple))
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_COLUMN),
+						 errmsg("column \"%s\" does not exist",
+								attname)));
+			attForm = (Form_pg_attribute) GETSTRUCT(atttuple);
+
+			/* Disallow use of system attributes in extended stats */
+			if (attForm->attnum <= 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("statistics creation on system columns is not supported")));
+
+			/* Disallow data types without a less-than operator */
+			type = lookup_type_cache(attForm->atttypid, TYPECACHE_LT_OPR);
+			if (type->lt_opr == InvalidOid)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("column \"%s\" cannot be used in statistics because its type %s has no default btree operator class",
+								attname, format_type_be(attForm->atttypid))));
+
+			attnums[nattnums] = attForm->attnum;
+			nattnums++;
+			ReleaseSysCache(atttuple);
+		}
+		else					/* expression */
+		{
+			Node	   *expr = selem->expr;
+			Oid			atttype;
+
+			Assert(expr != NULL);
+
+			/*
+			 * Disallow data types without a less-than operator.
+			 *
+			 * We ignore this for statistics on a single expression, in which
+			 * case we'll build the regular statistics only (and that code can
+			 * deal with such data types).
+			 */
+			if (list_length(stmt->exprs) > 1)
+			{
+				atttype = exprType(expr);
+				type = lookup_type_cache(atttype, TYPECACHE_LT_OPR);
+				if (type->lt_opr == InvalidOid)
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("expression cannot be used in multivariate statistics because its type %s has no default btree operator class",
+									format_type_be(atttype))));
+			}
+
+			stxexprs = lappend(stxexprs, expr);
+		}
 	}
 
 	/*
-	 * Check that at least two columns were specified in the statement. The
-	 * upper bound was already checked in the loop above.
-	 */
-	if (numcols < 2)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
-				 errmsg("extended statistics require at least 2 columns")));
-
-	/*
-	 * Sort the attnums, which makes detecting duplicates somewhat easier, and
-	 * it does not hurt (it does not affect the efficiency, unlike for
-	 * indexes, for example).
-	 */
-	qsort(attnums, numcols, sizeof(int16), compare_int16);
-
-	/*
-	 * Check for duplicates in the list of columns. The attnums are sorted so
-	 * just check consecutive elements.
+	 * Parse the statistics kinds.
+	 *
+	 * First check that if this is the case with a single expression, there
+	 * are no statistics kinds specified (we don't allow that for the simple
+	 * CREATE STATISTICS form).
 	 */
-	for (i = 1; i < numcols; i++)
+	if ((list_length(stmt->exprs) == 1) && (list_length(stxexprs) == 1))
 	{
-		if (attnums[i] == attnums[i - 1])
+		/* statistics kinds not specified */
+		if (list_length(stmt->stat_types) > 0)
 			ereport(ERROR,
-					(errcode(ERRCODE_DUPLICATE_COLUMN),
-					 errmsg("duplicate column name in statistics definition")));
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("when building statistics on a single expression, statistics kinds may not be specified")));
 	}
 
-	/* Form an int2vector representation of the sorted column list */
-	stxkeys = buildint2vector(attnums, numcols);
-
-	/*
-	 * Parse the statistics kinds.
-	 */
+	/* OK, let's check that we recognize the statistics kinds. */
 	build_ndistinct = false;
 	build_dependencies = false;
 	build_mcv = false;
@@ -313,14 +343,91 @@ CreateStatistics(CreateStatsStmt *stmt)
 					 errmsg("unrecognized statistics kind \"%s\"",
 							type)));
 	}
-	/* If no statistic type was specified, build them all. */
-	if (!requested_type)
+
+	/*
+	 * If no statistic type was specified, build them all (but only when the
+	 * statistics is defined on more than one column/expression).
+	 */
+	if ((!requested_type) && (numcols >= 2))
 	{
 		build_ndistinct = true;
 		build_dependencies = true;
 		build_mcv = true;
 	}
 
+	/*
+	 * When there are non-trivial expressions, build the expression stats
+	 * automatically. This allows calculating good estimates for stats that
+	 * consider per-clause estimates (e.g. functional dependencies).
+	 */
+	build_expressions = (list_length(stxexprs) > 0);
+
+	/*
+	 * Check that at least two columns were specified in the statement, or
+	 * that we're building statistics on a single expression.
+	 */
+	if ((numcols < 2) && (list_length(stxexprs) != 1))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				 errmsg("extended statistics require at least 2 columns")));
+
+	/*
+	 * Sort the attnums, which makes detecting duplicates somewhat easier, and
+	 * it does not hurt (it does not matter for the contents, unlike for
+	 * indexes, for example).
+	 */
+	qsort(attnums, nattnums, sizeof(int16), compare_int16);
+
+	/*
+	 * Check for duplicates in the list of columns. The attnums are sorted so
+	 * just check consecutive elements.
+	 */
+	for (i = 1; i < nattnums; i++)
+	{
+		if (attnums[i] == attnums[i - 1])
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("duplicate column name in statistics definition")));
+	}
+
+	/*
+	 * Check for duplicate expressions. We do two loops, counting the
+	 * occurrences of each expression. This is O(N^2) but we only allow small
+	 * number of expressions and it's not executed often.
+	 *
+	 * XXX We don't cross-check attributes and expressions, because it does
+	 * not seem worth it. In principle we could check that expressions don't
+	 * contain trivial attribute references like "(a)", but the reasoning is
+	 * similar to why we don't bother with extracting columns from
+	 * expressions. It's either expensive or very easy to defeat for
+	 * determined user, and there's no risk if we allow such statistics (the
+	 * statistics is useless, but harmless).
+	 */
+	foreach(cell, stxexprs)
+	{
+		Node	   *expr1 = (Node *) lfirst(cell);
+		int			cnt = 0;
+
+		foreach(cell2, stxexprs)
+		{
+			Node	   *expr2 = (Node *) lfirst(cell2);
+
+			if (equal(expr1, expr2))
+				cnt += 1;
+		}
+
+		/* every expression should find at least itself */
+		Assert(cnt >= 1);
+
+		if (cnt > 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_COLUMN),
+					 errmsg("duplicate expression in statistics definition")));
+	}
+
+	/* Form an int2vector representation of the sorted column list */
+	stxkeys = buildint2vector(attnums, nattnums);
+
 	/* construct the char array of enabled statistic types */
 	ntypes = 0;
 	if (build_ndistinct)
@@ -329,9 +436,23 @@ CreateStatistics(CreateStatsStmt *stmt)
 		types[ntypes++] = CharGetDatum(STATS_EXT_DEPENDENCIES);
 	if (build_mcv)
 		types[ntypes++] = CharGetDatum(STATS_EXT_MCV);
+	if (build_expressions)
+		types[ntypes++] = CharGetDatum(STATS_EXT_EXPRESSIONS);
 	Assert(ntypes > 0 && ntypes <= lengthof(types));
 	stxkind = construct_array(types, ntypes, CHAROID, 1, true, TYPALIGN_CHAR);
 
+	/* convert the expressions (if any) to a text datum */
+	if (stxexprs != NIL)
+	{
+		char	   *exprsString;
+
+		exprsString = nodeToString(stxexprs);
+		exprsDatum = CStringGetTextDatum(exprsString);
+		pfree(exprsString);
+	}
+	else
+		exprsDatum = (Datum) 0;
+
 	statrel = table_open(StatisticExtRelationId, RowExclusiveLock);
 
 	/*
@@ -351,6 +472,10 @@ CreateStatistics(CreateStatsStmt *stmt)
 	values[Anum_pg_statistic_ext_stxkeys - 1] = PointerGetDatum(stxkeys);
 	values[Anum_pg_statistic_ext_stxkind - 1] = PointerGetDatum(stxkind);
 
+	values[Anum_pg_statistic_ext_stxexprs - 1] = exprsDatum;
+	if (exprsDatum == (Datum) 0)
+		nulls[Anum_pg_statistic_ext_stxexprs - 1] = true;
+
 	/* insert it into pg_statistic_ext */
 	htup = heap_form_tuple(statrel->rd_att, values, nulls);
 	CatalogTupleInsert(statrel, htup);
@@ -373,6 +498,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 	datanulls[Anum_pg_statistic_ext_data_stxdndistinct - 1] = true;
 	datanulls[Anum_pg_statistic_ext_data_stxddependencies - 1] = true;
 	datanulls[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;
+	datanulls[Anum_pg_statistic_ext_data_stxdexpr - 1] = true;
 
 	/* insert it into pg_statistic_ext_data */
 	htup = heap_form_tuple(datarel->rd_att, datavalues, datanulls);
@@ -396,13 +522,42 @@ CreateStatistics(CreateStatsStmt *stmt)
 	 */
 	ObjectAddressSet(myself, StatisticExtRelationId, statoid);
 
-	for (i = 0; i < numcols; i++)
+	/* add dependencies for plain column references */
+	for (i = 0; i < nattnums; i++)
 	{
 		ObjectAddressSubSet(parentobject, RelationRelationId, relid, attnums[i]);
 		recordDependencyOn(&myself, &parentobject, DEPENDENCY_AUTO);
 	}
 
 	/*
+	 * If there are no dependencies on a column, give the statistics an auto
+	 * dependency on the whole table.  In most cases, this will be redundant,
+	 * but it might not be if the statistics expressions contain no Vars
+	 * (which might seem strange but possible). This is consistent with what
+	 * we do for indexes in index_create.
+	 *
+	 * XXX We intentionally don't consider the expressions before adding this
+	 * dependency, because recordDependencyOnSingleRelExpr may not create any
+	 * dependencies for whole-row Vars.
+	 */
+	if (!nattnums)
+	{
+		ObjectAddressSet(parentobject, RelationRelationId, relid);
+		recordDependencyOn(&myself, &parentobject, DEPENDENCY_AUTO);
+	}
+
+	/*
+	 * Store dependencies on anything mentioned in statistics expressions,
+	 * just like we do for index expressions.
+	 */
+	if (stxexprs)
+		recordDependencyOnSingleRelExpr(&myself,
+										(Node *) stxexprs,
+										relid,
+										DEPENDENCY_NORMAL,
+										DEPENDENCY_AUTO, false, true);
+
+	/*
 	 * Also add dependencies on namespace and owner.  These are required
 	 * because the stats object might have a different namespace and/or owner
 	 * than the underlying table(s).
@@ -583,87 +738,6 @@ RemoveStatisticsById(Oid statsOid)
 }
 
 /*
- * Update a statistics object for ALTER COLUMN TYPE on a source column.
- *
- * This could throw an error if the type change can't be supported.
- * If it can be supported, but the stats must be recomputed, a likely choice
- * would be to set the relevant column(s) of the pg_statistic_ext_data tuple
- * to null until the next ANALYZE.  (Note that the type change hasn't actually
- * happened yet, so one option that's *not* on the table is to recompute
- * immediately.)
- *
- * For both ndistinct and functional-dependencies stats, the on-disk
- * representation is independent of the source column data types, and it is
- * plausible to assume that the old statistic values will still be good for
- * the new column contents.  (Obviously, if the ALTER COLUMN TYPE has a USING
- * expression that substantially alters the semantic meaning of the column
- * values, this assumption could fail.  But that seems like a corner case
- * that doesn't justify zapping the stats in common cases.)
- *
- * For MCV lists that's not the case, as those statistics store the datums
- * internally. In this case we simply reset the statistics value to NULL.
- *
- * Note that "type change" includes collation change, which means we can rely
- * on the MCV list being consistent with the collation info in pg_attribute
- * during estimation.
- */
-void
-UpdateStatisticsForTypeChange(Oid statsOid, Oid relationOid, int attnum,
-							  Oid oldColumnType, Oid newColumnType)
-{
-	HeapTuple	stup,
-				oldtup;
-
-	Relation	rel;
-
-	Datum		values[Natts_pg_statistic_ext_data];
-	bool		nulls[Natts_pg_statistic_ext_data];
-	bool		replaces[Natts_pg_statistic_ext_data];
-
-	oldtup = SearchSysCache1(STATEXTDATASTXOID, ObjectIdGetDatum(statsOid));
-	if (!HeapTupleIsValid(oldtup))
-		elog(ERROR, "cache lookup failed for statistics object %u", statsOid);
-
-	/*
-	 * When none of the defined statistics types contain datum values from the
-	 * table's columns then there's no need to reset the stats. Functional
-	 * dependencies and ndistinct stats should still hold true.
-	 */
-	if (!statext_is_kind_built(oldtup, STATS_EXT_MCV))
-	{
-		ReleaseSysCache(oldtup);
-		return;
-	}
-
-	/*
-	 * OK, we need to reset some statistics. So let's build the new tuple,
-	 * replacing the affected statistics types with NULL.
-	 */
-	memset(nulls, 0, Natts_pg_statistic_ext_data * sizeof(bool));
-	memset(replaces, 0, Natts_pg_statistic_ext_data * sizeof(bool));
-	memset(values, 0, Natts_pg_statistic_ext_data * sizeof(Datum));
-
-	replaces[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;
-	nulls[Anum_pg_statistic_ext_data_stxdmcv - 1] = true;
-
-	rel = table_open(StatisticExtDataRelationId, RowExclusiveLock);
-
-	/* replace the old tuple */
-	stup = heap_modify_tuple(oldtup,
-							 RelationGetDescr(rel),
-							 values,
-							 nulls,
-							 replaces);
-
-	ReleaseSysCache(oldtup);
-	CatalogTupleUpdate(rel, &stup->t_self, stup);
-
-	heap_freetuple(stup);
-
-	table_close(rel, RowExclusiveLock);
-}
-
-/*
  * Select a nonconflicting name for a new statistics.
  *
  * name1, name2, and label are used the same way as for makeObjectName(),
@@ -731,19 +805,28 @@ ChooseExtendedStatisticNameAddition(List *exprs)
 	buf[0] = '\0';
 	foreach(lc, exprs)
 	{
-		ColumnRef  *cref = (ColumnRef *) lfirst(lc);
+		StatsElem  *selem = (StatsElem *) lfirst(lc);
 		const char *name;
 
 		/* It should be one of these, but just skip if it happens not to be */
-		if (!IsA(cref, ColumnRef))
+		if (!IsA(selem, StatsElem))
 			continue;
 
-		name = strVal((Value *) linitial(cref->fields));
+		name = selem->name;
 
 		if (buflen > 0)
 			buf[buflen++] = '_';	/* insert _ between names */
 
 		/*
+		 * We use fixed 'expr' for expressions, which have empty column names.
+		 * For indexes this is handled in ChooseIndexColumnNames, but we have
+		 * no such function for stats and it does not seem worth adding. If a
+		 * better name is needed, the user can specify it explicitly.
+		 */
+		if (!name)
+			name = "expr";
+
+		/*
 		 * At this point we have buflen <= NAMEDATALEN.  name should be less
 		 * than NAMEDATALEN already, but use strlcpy for paranoia.
 		 */
@@ -754,3 +837,29 @@ ChooseExtendedStatisticNameAddition(List *exprs)
 	}
 	return pstrdup(buf);
 }
+
+/*
+ * StatisticsGetRelation: given a statistics's relation OID, get the OID of
+ * the relation it is an statistics on.  Uses the system cache.
+ */
+Oid
+StatisticsGetRelation(Oid statId, bool missing_ok)
+{
+	HeapTuple	tuple;
+	Form_pg_statistic_ext stx;
+	Oid			result;
+
+	tuple = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(statId));
+	if (!HeapTupleIsValid(tuple))
+	{
+		if (missing_ok)
+			return InvalidOid;
+		elog(ERROR, "cache lookup failed for statistics object %u", statId);
+	}
+	stx = (Form_pg_statistic_ext) GETSTRUCT(tuple);
+	Assert(stx->oid == statId);
+
+	result = stx->stxrelid;
+	ReleaseSysCache(tuple);
+	return result;
+}